Polyurethane adhesive is desirable as an adhesive for book binding. The polyurethane adhesive cures to form a film that strongly adheres to the page edges exposed at the book block spine, but is pliable and permits easy opening of the book.
There have been problems with use of polyurethane adhesive (PUR) in book binding. Once the material is exposed to moisture in the atmosphere it cures, i.e., becomes cross linked, and will thereafter not return to liquid form as is possible with conventional hot melt book binding adhesives. Typically, adhesives are applied to a book block spine using a roller. The roller is immersed within a pan containing a quantity of liquid adhesive. The adhesive clings to the roller surface for transfer to book blocks passing over the top of the roller. As book blocks pass by and contact the top of the roller, the book block spines receive the adhesive from the roller surface.
This process of bathing the roller in adhesive is acceptable for conventional hot glue book binding adhesives placed in and returned to liquid form by heating. It will also work for the PUR adhesive as long as the process is not interrupted, i.e., as long as the bath of liquid PUR is continuously applied to the roller and then to the book block spine before cross linking occurs and as long as the bath of liquid PUR is continuously replenished. When the process is interrupted, however, as it invariably is, the PUR liquid starts to cure. In a short time, the PUR on the roller and in the pan begins to cure from exposure to atmospheric moisture. For PUR this curing is irreversible. Before the binding process can be restarted, the roller and pan may have to be cleaned of the PUR. This is a time consuming and highly undesirable occurrence that, heretofore, has been unavoidable with use of PUR in book binding.